Mirror Symmetry of Life Beata Zagórska-Marek
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Chapter Mirror Symmetry of Life Beata Zagórska-Marek Abstract Functioning in the Earth gravity field imposes on living organisms a necessity to read directions. The characteristic feature of their bodies, regardless unicellular or multicellular, is axial symmetry. The development of body plan orchestrated by spatiotemporal changes in gene expression patterns is based on formation of the vertical and radial axes. Especially for immobile plants, anchored to the substrate, vertical axis is primary and most important. But also in animals the primary is the axis, which defines the anterior and posterior pole of the embryo. There are many little known chiral processes and structures that are left- or right oriented with respect to this axis. Recent developments indicate the role of intrinsic cell chirality that determines the direction of developmental chiral processes in living organisms. The still enigmatic events in cambia of trees and handedness of phyllotaxis as well as plant living crystals are in focus of the chapter. Keywords: intrinsic cell chirality, charophytes, cambium, morphogenesis, figured wood, plant development, phyllotaxis, shoot apical meristem, snail shells, handedness, dislocations I see the mirror fairy tale of infinite reflections spinning out to weave no end.... Bolesław Leśmian “Prolog” 1. Introduction Polarized environment, from the beginning of life on our planet, imposes on living organisms a necessity to read directions. Light and gravity are the two most important oriented signals that come from well-defined sources. The response to these primary polar signals allowed for development of the second- ary, more sophisticated reactions to the network of other polar signals like gradients of chemical molecules or mechanical stresses. The signals of chemical and physical nature provide the extrinsic but also an intrinsic information for biological systems. One of the basic features of the organism developing in polar environment is its axiality. This brings in consequence following possibilities: 1) multiplication of repetitive units of the body and their special alignment along the axis (segmenta- tion, metamerism) and 2) deviations of structures from the axis to the left or to the right (development of L/R symmetry). The main axis in the motionless plants, fastened to the ground, is mostly verti- cal, extending between the apical and basal poles. The animal main axis, regardless its position in the gravity field, connects the anterior and posterior poles. The axis 1 Current Topics in Chirality - From Chemistry to Biology formation starts at the very early stages of development. The identity of segments formed iteratively along the axis in both plants and animals is genetically controlled and their evolutionary multiplication creates a great potential for morphological and functional diversity through many useful modifications. This process is in a sense similar to the effects of gene duplication on the molecular level. Subsequent emergence of L/R symmetry may be observed on all, hierarchi- cally different levels of body organization. Some general principles, like minimum energy rule, are universal in nature leading to the identical solutions on all these levels. Good example represent spherical geodesic shapes. The structure of carbon allotrope - C60 closed fullerene, is also present in coated endocytic vesicles rein- forced by the clathrin cage [1], in regularly sculptured surface of pollen grains [2] and in a cellular pattern on the surface of the plant paraboloid apical meristem [3]. Other universal basic forms are chiral helices and spirals commonly observed on molecular, cellular and organismal levels. They are of particular interest here because of distinct mirror symmetry they have, which is the main focus of this chapter. Chirality of many macromolecules: nucleic acids, proteins or cellulose fibers [4], coiled coils of collagen, or such structures as tubulin cytoskeleton, thickenings of plant cell wall, plant tendrils, spiral snail shells or narwhal tusks are but a few examples. Not only structures but also some developmental processes may be chiral. The apical cell divisions in moss gametophores [5], cell cleavage in the embryos of snails [6, 7] or lateral organ initiation on plant shoot apical meristem (SAM) proceed clockwise (CW) or counterclockwise (CCW). Two interest- ing problems may be addressed while considering chiral structures in biological systems – mechanism of their formation and proportion between the two chiral configurations. The aim of this chapter is to provide the readers with the overview of some examples of bio-chirality discovered over the years both in animals and plants. The stronger accent will be placed on the latter because they are less known and because they have always been in a focus of the author’s research. The mechanisms of many cases of mirror-symmetry presence in plants are yet to be elucidated. 2. Mirror symmetry on cellular level Cell chirality or handedness is a newly discovered phenomenon, which nowa- days is intensely studied, mostly in animal cells [7–10]. It is manifested in the presence of chiral structures within the cells but also in the cell behavior that may lead to directional movements or assuming L or R orientation of cell alignment. In animals it affects organs laterality [10], in plants results in development of spiral, helical or wavy patterns [3, 11–15]. During primary axis formation on the cellular level the polarity of the cell is manifested in an uneven distribution of receptors, ion channels and hormone carri- ers on plasma membrane, and internally in the ion currents and cytoskeletal fibers parallel to the developing axis but also in the polar distribution of ultrastructural components like cell organelles or nutrients. All these sophisticated processes have been investigated mainly in plant egg cells or fucoid zygotes [16–18] and animal oocytes [19, 20]. However, even in the integrated system of multicellular organism, singular cell polarity is often a case. In animal body the epithelial cells of intestines constituting a planar 2D barrier, have their polarity unified. It is manifested in nonrandom distribution of glucose transporters which facilitates the oriented tran- sepithelial sugar transport [21]. In plants the polar distribution of the auxin influx (AUX) and efflux (PIN) carriers in plasma membrane results in polar transport of the hormone between the cells [22]. In L1 layer of SAM auxin is transported 2 Mirror Symmetry of Life DOI: http://dx.doi.org/10.5772/intechopen.96507 acropetally, whereas inside of the plant body, in the provascular tissues and later in cambium, the hormone transport is basipetal. Change in the distribution of carriers and thus of the cell polarity redirects the transport, often affecting the directions of plant organ growth [23]. This, for instance, has been noted in gravitropic response of the roots [24]. Many elements of the cell ultrastructure are spectacularly chiral. In some green algae exemplified by multicellular filamentous Spirogyra or unicellular Spirotaenia and Chlamydomonas spirogyroides the chloroplasts are of considerable length, flat and ribbon-like. They assume helical course in the cortical cytoplasm of the cell. Not much is known about the chiral configurations of their coiling. Images available in various data bases suggest that in Spirogyra both configurations may be present in different filaments [25], in different cells of the same filament or even within the same cell [26]. However, the error resulting from improper focusing during microscopic observations cannot be excluded. The sample taken for analysis from the aquarium of the Botanical Garden of Wrocław showed under light microscope hundreds of cells of the same S configuration of chloroplasts coiling from the right to the left (Figure 1). Mechanisms by which the configuration is regulated remain undiscovered. Another clearly chiral component of the cell is basal body. In eukaryotic, plant and animal cells some identical structures bear different names although they look the same. Two centrioles of the centrosome, basal bodies or kinetosomes in the motile or ciliary epithelial cells have the same architecture. Composed of 9 triplets of microtubules (MT), overlapping on all available images either CW or CCW, they resemble a pinwheel toy. It is unclear, however, whether both configurations, being a mirror-image of one another, are indeed present in all different types of the cells. Transmission electron micrographs (TEM) of tangentially sectioned cell surface show that in a particular cell all basal bodies underlying cilia are of the same chiral- ity [27]. However, unless it is clearly stated like in [28], it is not known whether Figure 1. The same filament of Spirogyra photographed under fluorescent microscope at different optical levels: at its upper surface (left) and well below, close to its opposite side (right). S helix of the chloroplast, visualized here by the red autofuorescence of chlorophyll, may be falsely interpreted as Z, when due to the changing focus is watched from the inside of the cell. 3 Current Topics in Chirality - From Chemistry to Biology basal body is seen on TEM from the surface of the cell or from its inside. This is the reason for chiral configurations of basal bodies being uncertain. The image of Paramaecium micronucleatum by Dennis Kunkel [27] shows CCW overlapping triplets, whereas in Paramecium tetraurelia [29] the triplets overlap CW. In flagellar apparatus of Chlamydomonas or Acrosiphonia